The scytale (ancient Greek σκυτάλη skytálē, “stick,” “rod”) is the oldest known military encryption method. A stick of a certain diameter was used for encryption.
To compose a message, the sender wound a parchment ribbon or strip of leather helically around the skytál, wrote the message along the length of the stick on the ribbon, and then unwound it. The ribbon without the staff is delivered to the recipient. If the tape fell into the wrong hands, the message could not be read because the letters appeared to be arranged randomly on the tape.
The correct recipient of the tape could read the message with an identical scytale (a stick with the same diameter).
The diameter of the rod is thus the key in this encryption method.
The scytale belongs to the transposition ciphers.